Bicycle Mechanics - Why does my seat post keep coming loose?
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I have a Trek "Mountain Trek" bike. Recently, the seat post keeps loosening and then foom! The seat post shoots down the post tube.
I check the seat post collar lever-clamp and it is still tight in position. That is, it didn't somehow pop open or come loose.
To remedy, I loosen the lever-clamp give it a couple of clockwise spins, and push it back down in to tight position. A couple of blocks later, the seat starts rocking side-ways and I know what is coming next; FOOM! Again.
Any ideas what the heck is going on?
12-21-00, 01:18 AM
Some of the clamps used by trek are not very effective. Tighten the allen screw, on the location side of the clamp, until the clamp just manages to close. Alternatively, tighten the allen screw with the clamp closed.
12-21-00, 06:08 AM
If you do not often remove or lower the seat, you can replace the seatpost skewer with a bolt and nut. Your LBS should have just the right ones, or you could take the skewer to a hardware store and match it up. (Don't waste time at a mega-builder-supply store, go to an old fashioned nut & bolt hardware store.)
I did this with my Trek "rainbike," after the skewer rusted and broke. This mod will shave a few grams of useless weight from your bike, as well.
I take it you didn't change seat post.
Try another QR or a simple seat bolt. If they work, then you're done.
If you have tried other bolts and QRs and still comes loose then you will need a new seapost. Either the post is not the right diamter, even though the post says or the frame is a bit bigger in diameter than it should be.
I doubt this is the case, but either way you should know.
Thanks for all the suggestions.
I am using the original seat post. Interestinly, it is tight on some parts of the post, and loose on others. Maybe the post was crushed or compressed in some parts? If so, it can't be seen with the naked eye.
Anyway, the ends of the clamp collar are touching each other, so it isn't possible to tighten it any more and still the seat post comes loose. It is extremely frustrating.
It is an extra long post which I need, so it would be good if I could keep it.
My plan now is to slip some shim material down the post collar. Do they sell shim material for this purpose? I am thinking about using an aluminum can for shim-stock, but am afraid that it might permanently unite with the ChroMoly frame.
12-25-00, 02:05 PM
The seat post you have is TOO SMALL in diameter, either the shop or Trek F***ed up. I've seen it happen before with other brands and shops. Have your seat tube inner diameter measured and get the appropriate size post, (this is is the best long term solution). You can shim it if you need to wait, try to get some shims made for the purpose, the aluminum cans are a pain. If you have a decent shop they should replace your post for free and if Trek did send the wrong size they will reimburse them. This "post Problem" is their fault not yours, if they're not "cool" about it get a new shop and write Trek a letter.
[Edited by pat5319 on 12-25-2000 at 04:12 PM]
So it is all original items and all the sudden it will not tighten? That is very weird indeed.
The post is not bend or anything or it wouldn't slide in at all. So no worries there. And no one enlarged the opening.
Hopefully you can replace the clamp on your Trek and it is not cheaply welded on. Simply buy a replacement clamp. If this doesn't work then buy another post. You can try shimming as many people do with this and stems and it will not be a problem but I always recommned in order to not get sued her ein America to simply buy another post and try that. After you tried a new clamp. Clamp is much cheaper.
Thanks for all the suggestions.
It sounds like I might have to get a new post. It is a shame to waste the good existing post - silly as that sounds. Yes, it is the original post, but like I said, some parts of the post is too small and some are JUST big enough. Maybe the post was barely the right size to begin with and now, after the collar has stretched a bit, or the post has compressed, it is too small.
The existing collar is part of the seat tube, so I can't simply replace the collar.
Hmmm... what a shame and what an inconvenience.
Curious to know the post brand? Many times on these mass produced bikes the tubes are not exactly the right diameter. I have seen many that are usually too small thus all we have to do here is grind a bit. However too large is always a problem. However there are many shims out in the market in many sizes for this same problem. Of course a COKE can has been the solutions for many riders since the early days of the can.
Well, Fellers, for the time being, I made a shim out of a steel Campbells soup can.
Since the seat post was steel and the frame is ChroMoly, I thought it would be risky to use aluminium material as a shim because it might react/fuse with the other metals making it difficult to adjust or remove the post or the shim in the future.
For the record, an old timer showed me how to make a good shim. You can't just try to slip some shim stock into the collar inner diameter or onto the post outer diameter. If you try it, you will find that the shim stock will just be shifted around to to the friction of the surfaces and it is almost impossible to get it right.
What you do is make a bunch of little cuts about 3/8" deep and about 1/4 " apart across one edge of the shim stock. Then, as you roll the shim stock to fit inside the collar, you fan out these cut "tabs" so that they fan to the outside of the newly formed shim tube. These tabs catch the side of the collar so that the shim stock tube doesn't fall down the post tube as you push the seat post into it.
So far, it is working fine. I suspect that the steel soup can stock may start to rust eventually despite the coating of clear-coat I gave it.
01-06-01, 12:37 PM
The last commercial shims i saw were steel, only crome plated. They also rust, i think you are on the right track though. a seat post is a terrible thing to waste.!!!
01-07-01, 12:07 AM
Hey Mike, Did you grease all of the surfaces of the shim, seat-pin(post) and inside of the tube? If not they'll rusttogether in no time
Yes, I did grease all surfaces. Anybody who has struggled with trying to get a frozen seat post or gooseneck loose will never forget to grease those parts!
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